Although normal cells proliferate by the highly controlled activation of growth factor receptor tyrosine kinases (RTKs) by their respective ligands, cancer cells also proliferate by the activation of growth factor receptors, but lose the careful control of normal proliferation. The loss of control may be caused by numerous factors, such as the overexpression of growth factors and/or receptors, and autonomous activation of biochemical pathways regulated by growth factors. Some examples of RTKs involved in tumorigenesis are the receptors for epidermal growth factor (EGFR), platelet-derived growth factor (PDGFR), insulin-like growth factor (IGFR), nerve growth factor (NGFR), and fibroblast growth factor (FGF). Binding of these growth factors to their cell surface receptors induces receptor activation, which initiates and modifies signal transduction pathways and leads to cell proliferation and differentiation.
Members of the epidermal growth factor (EGF) receptor family are particularly important growth factor receptor tyrosine kinases associated with tumorigenesis of epidermal cells. The first member of the EGF receptor family to be discovered was EGFR, which is expressed on many types of tumor cells. EGFR has been found to be involved in regulation of tumor cell division and growth, repair and survival, angiogenesis, invasion and tumor metastasis.
EGFR is a 170 kD membrane-spanning glycoprotein with an extracellular ligand binding domain, a transmembrane region and a cytoplasmic protein tyrosine kinase domain. Examples of ligands that stimulate EGFR include epidermal growth factor (EGF), transforming growth factor-α (TGF-α), heparin-binding growth factor (HBGF), β-cellulin, and Cripto-1. Binding of specific ligands results in EGFR autophosphorylation, activation of the receptor's cytoplasmic tyrosine kinase domain and initiation of multiple signal transduction pathways that regulate tumor growth and survival. The EGFR pathway also influences production of various other angiogenic factors, such as VEGF and basis fibroblastic growth factor (bFGF), in tumors.
Growth factors that activate EGFR are also thought to play a role in tumor angiogenesis. Angiogenesis, which refers to the formation of capillaries from pre-existing vessels in the embryo and adult organism, is known to be a key element in tumor growth, survival and metastasis. It has been reported that EGFR mediated stimulation of tumor cells leads to increased expression of the angiogenic factors vascular endothelial growth factor (VEGF), interleukin-8 (IL-8), and basic fibroblast growth factor (bFGF), which can lead to activation of tumor-associated vascular endothelial cells. Stimulation of tumor-associated vascular endothelial cells may also occur through activation of their own EGF receptors, by tumor produced growth factors such as TGF-α and EGF.
It has been reported that many human tumors express or overexpress EGFR. Expression of EGFR is correlated with poor prognosis, decreased survival, and/or increased metastasis. EGFR, because of this involvement in tumorigenesis, has been specifically targeted for anticancer therapies. These therapies have predominantly included either a monoclonal antibody that blocks binding of ligand to the extracellular domain of the receptor or a synthetic tyrosine kinase inhibitor that acts directly on the intracellular region to prevent signal transduction.
For example, Cetuximab MAb (ERBITUX®) is a recombinant, human/mouse chimeric, monoclonal antibody that binds specifically to the extracellular domain of the human EGFR. Cetuximab is an EGFR antagonist, which blocks ligand binding to EGFR, prevents receptor activation, and inhibits growth of tumor cells that express EGFR. Cetuximab has been approved for use in combination with or without irinotecan in the treatment of patients with epidermal growth factor receptor-expressing, metastatic colorectal cancer who are refractory or can not tolerate irinotecan-based chemotherapy. Cetuximab has also been shown to be effective for treatment of psoriasis.